The energy budget control in optical telecommunication networks is a key issue to reduce the implementation cost, optimize the performance and decrease the number of components and the size of systems used. For example, an optical telecommunication line needs more optical amplifiers if the losses are too high. It is therefore very important to choose optical components (e.g. variable optical attenuator (VOA), add/drop filter, etc.) with minimal losses when they are used in their transparent state.
Traditionally, telecommunication signal processing components are built using well-known techniques of semiconductor, mechanical or micro mechanical industries that are usually well adapted for free-space applications, where light is brought out from the fiber. However, it is well known that when the light is coupled out of a fiber to be transformed (filtered, attenuated, dropped, dispersion compensated, etc.) then there are always higher losses than if the light remains in the original communication fiber. Presently, among the efforts made to improve the effectiveness of the network, in-guide tuning of optical properties (light remains in the original waveguide while being transformed) appears to be a more suited solution for the fabrication of low loss optical components. The main underlying principle here is the use of the evanescent part of the signal to affect it dynamically.
The material issue appears to be inherently involved in this development process since it must provide media that is able to dynamically alter the optical properties of the guiding component. Composite organic materials, including polymers, liquid crystals, oils, surfactants, etc., are good candidates to address this paramount problem. This class of materials often provides low-cost fabrication techniques and is easy to manipulate in comparison with complex and costly operations needed to fabricate inorganic semiconductor components (vacuum deposition, very high curing temperature, fragile solid crystal growth, etc.).
Among the most interesting organic materials, liquid crystal materials and/or mixtures have demonstrated high reliability, excellent performance and cost-effectiveness in display technologies. Presently, LCs are considered as promising materials for the development of intelligent (i.e., dynamically reconfigurable) optical telecommunication networks. For example, LC displays (LCD's), which consist of a LC planar cell capable of changing the polarization state of light, may be used as a part of variable optical attenuators, optical switches, optical routers, tunable filters and polarization control devices. This technology is taking the advantages of the huge technical knowledge and know-how developed in the mature LCD industry. However, the LC devices used always remain in the free space geometry where the light is travelling in the direction perpendicular to the cell substrates. This fact requires that the light must be out-coupled from the waveguide, propagating in free space (through electrodes, etc.) and consequently suffering losses.
In addition, such micro LCD's do not provide an appropriate solution in terms of compact integration. By way of reference, U.S. Pat. No. 5,963,291 (Wu et al.) granted on Oct. 5th, 1999 describes an optical attenuator in which a LC cell acts as a voltage controlled polarization transformer. This polarization modulation is in turn transformed into an intensity modulation using appropriate static birefringent elements. In this invention, light first comes out of the fiber, passes through optical elements and returns into the output fiber, thereby featuring the mentioned above free space modulation mode.
Using LCs in waveguiding geometries seems to be a better way to combine the advantages of the LC (low cost, low loss and power consumption, strong electro-optic effect, short response time, high reliability, etc.) and the specific needs of integration (miniaturization, heatless operation, etc.). U.S. Pat. No. 6,285,812 (Amundson et al.) granted on Sep. 4th, 2001 describes a device where the light is guided in a special optical fiber having a LC core surrounded by a glass cladding. Light propagation properties are altered using appropriate electrode assembly and corresponding electric field geometry to periodically change the effective refractive index of the core material creating a tunable grating-like structure in the waveguide. The light remains in the fiber and the losses arise mainly from the index profile perturbations of the modified optical fiber. In this particular case however, additional losses may arise from linear and nonlinear scattering (since the greatest part of the mode is propagating in the LC medium), from polarization dependant losses (PDL) or from a mode profile and index mismatch with the next communication element (e.g., a standard SMF-28 fiber) in which the device is integrated. Thus, the splicing losses and nonlinearities along with the exotism of the fiber used and its cost are among numerous drawbacks of this solution. However, this idea demonstrates an application where LCs are used to control an optical signal in a guided regime.
Problems associated with the use of LCs in guided geometries, such as electro-optic cladding elements (see FIG. 1 identified as “Prior Art”), reside when the guiding surfaces/interfaces cannot be treated with conventional methods for example those employed in the LCD industry, such as ionic deposition, polymer mechanical rubbing or anisotropic UV curing, etc. to fix preferred orientation of LC director at these surfaces. As an example to demonstrate the difficult adaptation of standard techniques to special geometries is the problem of reduced diameter cylindrical waveguides (e.g. etched or tapered optical fibers) with an overall diameter typically at the order of 1 to 10 μm. The relative fragility of these structures makes it impossible to use standard LC aligning techniques (especially mentioned above contact methods). In contrast, planar waveguides are better adapted to conventional aligning methods, but are not easily adaptable to communication networks using optical fibers. It is worth mentioning that it is one of the objects of the present invention to overcome this problem.
LC materials that are synthesized to optimize the performance of traditional LCD's mainly address parameters like the birefringence Δn, defined by the difference of the extraordinary and ordinary refractive indices of the material ne−no, the dielectric anisotropy Δε, the viscosity γ, the elastic constants Ki and the nematic temperature range ΔTN. At the same time, for the guided-mode modulation applications, the LC material may be close to wave guiding boundaries, which may not be treatable with the same processes as for the LCD's. For example, United States laid-open patent application No 2003/0103708-A1 (Galstian et al.) published on Jun. 5th, 2003, describes a tunable waveguide where the LC is employed as an electro-controllable cladding of a cylindrical waveguide (fiber), confined in a planar cell geometry or in a cylindrical capillary. This application is represented in FIG. 1. The waveguide (fiber) is assembled in a cell, which is filled with the electro-optic material.
Preferably, the waveguide employed in U.S. patent application laid-open No 2003/0103708 A1 (Galstian et al.) is a commercially available optical fiber (e.g. SMF-28 or UV sensitive fibers) where the refractive indices involved are higher than the refractive indices of organic compounds forming LC mesophases. Also, in order to interact with the guided mode, the LC cladding must be close enough to the waveguide core, leading to an overall waveguide size of a few micrometers, typically. This may be achieved by etching, polishing or otherwise removing a part of the fiber or by fiber tapering. Deposition of a polymer film at the surface of this structure and its rubbing may not be appropriate to align LCs in the preferred direction without damaging the waveguide. Additionally, the polymer would become an important element of the guiding structure and its physical and optical properties (thickness, uniformity of the layer, refractive index, absorption, scattering, homogeneity, etc.) would become hardly controllable parameters. This is also a paramount issue for all other known surface treatment processes because of the very small size, special form of the waveguide (cylindrical geometry, etc.) and its fragility. Thus, the use of LC in guided mode modulation geometries requires alternative materials and aligning techniques.
U.S. Pat. No. 5,190,688 (Sage et al.) granted on Mar. 2nd, 1993 describe a means of reducing the refractive index of a liquid crystal mixture that facilitates the formulation of mixtures for use in a wider range of devices such as liquid crystal displays operating in an NCAP (Nematic Curvilinear Aligned Phase) mode and tunable planar or fiber optical waveguide applications. In the latter case, it is therefore assumed that the preferred geometry used is still a planar configuration which dictates that tunable optical fiber devices only enclose side-polished fibers often called half-couplers. Other configurations are not specifically addressed because of the tricky problem of the LC anchoring. Planar devices may be treated using well-known methods borrowed from the LCD technology. However, in the particular case of LC-based tunable optical waveguides, the authors of this U.S. Pat. No. 5,190,688 do not discuss the key issue of the anchoring while it remains a non-evident problem even in planar geometries because any surface treatment should affect the waveguide optical properties (e.g. a polymer film coated as an overlay layer on a planar lightwave circuit should become a part of the optical modelling of the device).
Optical propagation in waveguide devices has another key difference with respect to LCD's: light propagates along the waveguide axis and not through the thickness of the LCD cell. The light propagation length in LC is thus much longer in the guided mode (3 to 4 orders of magnitude) than in the LCD's. Hence, the major material synthesis challenges here would be the adjustment of the absolute ordinary refractive index value and the LC-to-waveguide anchoring properties. As described in the United States patent application No U.S. 2003/0103708 A1 (Galstian et al.), the refractive index of the outer electro-controllable cladding must be tuned near to the effective refractive index of the waveguide to obtain phase and/or amplitude modulation. The amount of the phase shift or attenuation of the signal depends directly on the interaction length (the part of the fiber where light interacts with the electro-controllable cladding) that is typically in the range of millimeters to centimeters. However, the difference, δ, between the core and cladding refractive indices ncore−ncladding is usually very small, being in the order of 10−3. On the other hand, LC materials may have a birefringence value as high as Δn=0.3, and the minimum value obtainable among commercially available mixtures is around Δn=0.035 (FIG. 2). To avoid the difficulties of fine tuning (variation of the external controlling field amplitude required to induce a corresponding refractive index variation) of the optical propagation properties of the device, it is desirable to have a LC with Δn=ne−no comparable but greater than δ, and it is also desirable to have an ordinary refractive index no, which is lower than the refractive index of the waveguide cladding (e.g. fused silica). Indeed, FIG. 3 describes the general working principle of the in-guide phase tuning or signal attenuation in the particular case of the electro-optic liquid crystal materials where, in particular, the response time (the time required to reorient the molecules from the elastic equilibrium to the prescribed orientation) depends on the applied electric field.
It is known that there is certain relation between the birefringence and the absolute refractive index of nematic LC materials. Such relationships are represented in FIG. 2. The graph is a collection of experimental values (Δn, ne) for commercially available nematic LC mixtures as well as pure nematic compounds. It indicates a strong correlation between the quantities Δn and ne. The values are measured at a visible wavelength (λ=589 nm) and mainly at room temperature (20° C.) where the mixtures exhibit a nematic mesophase. One can easily see that low refractive index corresponds to low birefringence, in perfect agreement with the guided mode modulation requirements. As mentioned above, tunable waveguides made of silica (such as standard optical fibers) require an external medium having a minimum refractive index (which is the ordinary refractive index no in the case of positive birefringence LC materials) that is near and preferably lower than the refractive index of the silica glass in the desired spectral band. Usually, the dispersion of the transparent organic materials forming LC mixtures is comparable to that of silica and the relative difference between their refractive indices at a visible wavelength may be reported to the near infrared spectral area. The limit curve displayed in FIG. 2 indicates the boundary between LC compounds that satisfy or not this condition. Only a few existing mixtures are suited for the specific task of tuning silica waveguides' guiding properties (wavelength selectivity, phase, amplitude, polarization, etc.) but some of their key properties are not adapted for that application.
The media available from the prior art do not allow the required advantages to be achieved simultaneously. To summarize, the composite liquid crystalline mixture must simultaneously comply with the following requirements:                the nematic range, which defines the main operation condition, must preferably comprise a well-defined temperature interval (e.g. between −10 to +70° C.);        the optical properties of the mixture must be adjusted to have at least one of the refractive indices below the refractive index of the silica;        the anchoring energy of the mixture at the silica waveguide surface must be negligible so the director could align along the desired direction by means of geometrical elastic energy potential and external control fields;.        the anchoring properties at other aligning interfaces, such as rubbed polymer should remain reliable; and        the mixture has to be chemically and thermally stable, and must not degrade when submitted to electromagnetic radiation.        
In view of the above, it is obvious that there is a need for a specifically designed electro-controllable material.